1. Field of the Invention
This invention relates to orbital drive systems and more particularly to a counterweight construction that significantly reduces inertia caused by the mass of the counterweight.
2. Description of the Prior Art
Orbital drive systems, that is systems in which one or more cams or other structures move about an orbital path offset from the center of rotation of the drive shaft are well known in the art. Speed reduction mechanisms making use of epicyclical movements and gears having a perimeter defined by an epitrochoid curve have long been known. The magazine Design News in its Aug. 18, 1961 issue describes a single-stage cycloidal cam that forms the basic of a speed reducing mechanism. A similar structure is shown in U.S. Pat. No. 1,682,563 to Hill. Corresponding devices were previously known that made use of spur gears. U.S. Pat. No. 2,250,259 to Foote, Jr. describes such a heliocentric unit. Other related mechanisms are described in U.S. Pat. Nos. 3,429,393; 3,144,791; 3,783,712; and 3,144,791. Some of these mechanisms incorporate two stages so that the output shaft is in line with the input shaft.
My earlier U.S. Pat. No. 3,998,112 describes a speed reducing mechanism in which a series of rollers are equally spaced in a circle on a disk to perform operationally somewhat like an internal ring gear. Mounted for eccentric movement and simultaneously engaging each of the rollers is an orbital rotor having a perimeter defined by an epitrochoid curve with a number of lobes equal to one less than the number of rollers. The rotor is mounted for free rotation on an eccentric keyed to a drive shaft. Rotation of the drive shaft causes the rotor to move orbitally and to rotate with a speed reduction equal to the reciprocal of the number of lobes on the rotor. A similar structure may serve as a second stage with a second orbital rotor driven by the first rotor and moved orbitally by the same eccentric. The second stage provides a further speed reduction and an output drive, connected to the disk supporting the rollers in the second stage, that is in alignment with the input drive shaft.
An improved drive system with significantly less backlash is described in my co-pending application Ser. No. 06/344.886. filed Feb. 1, 1982, entitled Speed Reducer and Method for Reducing Backlash. Each of these units makes use of a pair of counterweights secured to its drive shaft to prevent excessive vibration that would otherwise be caused by the orbital movement of the two cams mounted for free rotation on an eccentric driven by a rotating shaft. Those counterwights, which rotated with the drive shaft, were effective in minimizing vibration, but added significantly to the inertia of the system.